DENIERS SAY:

What about 400 million years ago, when CO2 was five times higher than it is right now?

SCIENCE SAYS:

Looking back at naturally-caused global warming hundreds of millions of years ago is a nice history lesson, but it tells us little about man-made global warming today.

Because of pollution from dirty energy like coal, there is more carbon dioxide in the atmosphere now than in nearly a million years. More than 400 million years ago, however, carbon dioxide concentrations were much higher than today, and the world was mostly a lot warmer as well. Back then, the warming was indeed due to natural causes.

What does that tell us about climate change today? Not much. The world was a completely different place 400 million years ago. The continents were in different positions. Life was almost entirely restricted to the oceans. Trees didn’t exist. Humans didn't exist. In other words, not all pre-historic climate facts are relevant to the modern climate crisis.

Scientists have very accurate records that clearly show we have more carbon dioxide in the atmosphere today than any time in hundreds of thousands of years. If we go back a lot further in time, we can find instances when the concentration of carbon dioxide in the air was far greater than it is today. But many characteristics of the early Earth differ from the Earth we know today. For instance, the continents were in different locations and the sun was less hot than today’s sun. Simply put, it was a different world.

Many skeptics have suggested that since early glacial periods sometimes occurred with high levels of carbon dioxide, it must mean carbon dioxide is not a potent greenhouse gas. In fact, a non-scientist even testified to this effect to the U.S. Congress. This testimony and the erroneous conclusions reflect a nearly complete misunderstanding of the Earth’s climate; it was the subject of this response (PDF).

Before humans, the amount of carbon dioxide in the atmosphere was controlled by natural factors. A process called chemical weathering pulled carbon dioxide out of the air. At the same time, volcanic emissions injected carbon dioxide into the air. When these processes are balanced, the amount of carbon dioxide stays relatively constant. When one process becomes stronger, the amount of carbon dioxide can change.

When the Earth became colder in the past, the weathering process slowed down. As a consequence, the amount of carbon dioxide given off by volcanoes was not absorbed by weathering. Therefore, the carbon dioxide in the atmosphere increased. Over time, carbon dioxide increased so much that all the glaciers melted. The climate then shifted very quickly from cold to very hot. In fact, this resulted in what is sometimes called hothouse Earth.

So, in fact, not only does the presence of carbon dioxide coincident with glaciers not disprove the impact of carbon dioxide, but the transition from an ice-covered Earth to a hot Earth relies upon greenhouse gas theory.